The present invention relates generally to processing seismic data and more particularly to a method for imaging multicomponent seismic data to obtain better depth images of the earth's subsurface geological structure as well as better estimates of compressional and shear wave interval velocities.
Seismic exploration in its simplest form comprises imparting seismic energy into the earth with a seismic source (e.g., dynamite, vibrator, airgun, etc.) and recording the earth's response thereto with receivers (e.g., geophones, transducers, hydrophones, etc.). The seismic data recorded by such receivers are commonly referred to as seismic signals or seismic-trace signals. A plurality of such seismic signals obtained from a selected region of interest can be used to form seismograms or seismic sections to aid the geophysicist in interpreting the earth's subsurface geological structure. More recently, multicomponent seismic data acquisition techniques have been developed for recording sets of multicomponent seismic signals.
Generally, multicomponent seismic data acquisition techniques comprise imparting seismic energy into the earth with seismic sources having one or more linearly independent lines of action (i.e., known vector displacement or traction) and recording the earth's response with sets of receivers having at least two linearly independent lines of action. In practice, multicomponent seismic data can be acquired using seismic sources having horizontal transverse, horizontal radial and vertical lines of action and recording the earth's response to the imparted seismic energy with receivers having horizontal transverse, horizontal radial and vertical lines of action. Unfortunately, by common practice such sources and receivers are referred to as horizontal shear (SH), vertical shear (SV), and compressional (P) sources and receivers, respectively. This terminology is both inexact and misleading since sources and receivers do not directly impart or record such wavefields (i.e., horizontal shear, vertical shear or compressional) but rather impart and record known vector displacements from which it is possible, by partitioning, to separate the various wavefields.
Seismic data is usually processed in the common midpoint (CMP) format. Typically, part of the processing of the seismic data can include the step of migrating stacked CMP gathers of seismic signals. Migration is the process of placing reflection events recorded in the seismic data at their proper spatial location. Migration is especially important for correctly plotting dipping bed reflections in their true spatial position rather than midway between the activated source and the recording receiver. When the migration process is applied to CMP stacked seismic data, serious errors can result when the subsurface geological structures are complex (i.e., not simply a series of parallel, horizontal bedding planes). The errors introduced are the result of CMP processing of seismic data from complex subsurface geological structures because the reflection points are no longer common nor midway between the seismic source and receiver. Among the more significant limitations of such processing schemes are the inability to determine interval velocities in complex structures and the consequent inability to produce optimum depth migrated seismograms. Additionally, such processing is inadequate to separate and process coupled compressional and shear wave reflections in the recorded seismic data. Moreover, when mode converted wavefields are recorded, CMP stacking is extremely noise sensitive. Exemplary of such migration technique when applied to CMP formatted seismic data is shown by Vreugde in U.S. Pat. No. 4,110,729. The method of depth imaging multicomponent seismic data of the present invention is adapted to overcome these limitations.